am not sure about lcd screen protectors from canon, but sony do have them and all you have to do is specify lcd screen size
just got mine for my sony dsc t70, 3″ screen size
and believe me, i needed it pretty badly cause it’s touchscreen operated and i gladly parted with $7 for that
they also have them for 2.5″ & 3.5″ … that i know of

not sure about your screen size, but i guess it’s 2.7″ > in which case if sony does not have a 2.7″, you can buy the 3″ & cut out the extra portion with a pair of fine scissors

just sharing my limited knowledge, cause there’s no reason why canon should not be having it … just that i do not know

as far as cases, plenty many optionals available and sure something should fit just right … and all the best on that

edit:
supplies and accessories for your sx100
http://estore.usa.canon.com/cigSeeAllSuppliesAndAccessories.asp?product=2091B001&src=sna_estore&WT.mc_id=C000000
note that the Deluxe Soft Case PSC-100 $19.99 is compatible with the sx100 only … so just right that !

and as far as your lcd screen protector, 2.5″
closest i could locate was from sony india
http://www.sony.co.in/product/pck-l25/sku/pck-l25%2B%2B%2B%2B%2Bae?site=hp_en_IN_i

[affmage source="ebay" results="15"]Lcd Protector[/affmage]

get one on eBay. You can get like 5 screen protectors for about $1.00 and it is FREE SHIPPING

[affmage source="ebay" results="15"]Protector Amazon[/affmage]

Clean it off with eye glass cleaner, make sure all dust hair and smudges are gone because these can cause bubbles when applying the film. Try not to get your fingerprint on the film, then use something to squeegee out the bubbles after you apply it.

Apple it along the top first then go down.